1. Field of the Invention
The present invention relates in general to a vibration damper attached to a subject member to damp vibrations of the subject member, and more particularly to such a vibration damper of active control type adapted to actively or positively damp the vibrations.
1. Discussion of the Related Art
A dynamic damper is widely known as means for damping vibrations of a subject member which is likely to be vibrated, such as a body of an automotive vehicle. Recently, there have been proposed vibration dampers of active control type adapted to positively damp or control the vibrations of the subject member by applying oscillations to the subject member, in an attempt to obtain an enhanced damping effect. Examples of such active control type vibration dampers are disclosed in JP-A-3-292219 and JP-A-6-235438, wherein a mass member is connected through a spring member to an attaching member attached to the subject member, so that the mass member, spring member and attaching member cooperate to constitute a vibration system. The vibration dampers disclosed in these publications include an electromagnetic drive mechanism for applying an oscillating force to the mass member of the vibration system to thereby apply a large oscillating force to the subject member through the vibration system.
However, such conventional vibration dampers suffer from an inevitable increase in the size and weight, due to the provision of the electromagnetic drive mechanism.
The electromagnetic drive mechanism not only requires a coil and a magnet, but also suffers from problems of complicated construction and difficult and costly manufacture, due to a need of positioning those coil and magnet with high accuracy in order to assure high stability of the drive force generated.
Further, the conventional vibration dampers using the electromagnetic drive mechanism are not satisfactory in the operating reliability and durability due to the complicated construction, and tend to have difficulty in dealing with any defects which may take place.